NOVEL SYNTHESIS METHODS FOR PREPARATION OF CERIA ABRASIVES FOR CHEMICAL MECHANICAL PLANARIZATION APPLICATIONS IN SEMICONDUCTOR

PROCESSING

By

OH MYOUNG HWAN

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2010

1

2010 Oh Myoung Hwan

2

To my lovely wife, Minyoung and daughter, Yujin

3

ACKNOWLEDGMENTS

I am sincerely appreciative of my advisors, family members, colleagues, and

friends whose support made it possible for me to complete my doctoral studies and

research.

At this time, I would like to take a moment to acknowledge my professor, Dr. Rajiv

K. Singh for challenging and guiding me through this research and for always being

present as a mentor in science and in life. His support, guidance, caring, and patience

allowed me to complete this research and to grow as a scientist. Also, I would like to

acknowledge my committee, Dr. Stephen Pearton, Dr. David Norton, Dr. Hassan El-

Shall, and Dr. Chang-Won Park, for their advice and support. In addition, special thanks

are extended to the faculty and staff at the Particle Engineering Research Center and

Major Analytical Instrumentation Center, especially Dr. Valentine Craciun, Eric Lambers,

and Kerry Siebein for always preparing my samples to exact specification and for

providing access to their instrumentation.

I am deeply indebted to my family members for all their help which have truly

made it possible for me to reach my goals and obtain my dreams. I would like to thank

my grandmother, Dalwha Choi, for her life lessons and encouragement. I would

especially like to thank father and mother, Bonggeun Oh, and Jungsoon Yoo, for their

great love and full support for me. I would also like to thank my father-in-law and

mother-in-law, Soonbong Jang and Hyeonsook Lee, from the bottom of my heart for

generous love and encouragement. I thank my brother, Kyounghwan Oh, for his deep

affection for Yujin. I extend special thanks to brother-in-law and his family, Junsoo Jang,

Yeonok Park, Uin Jang, Soyun Jang. Additionally, I would like to acknowledge to

president of LG Chem Research Park and Director of Corporate R&D center, Dr. Jin-

4

Nyoung Yoo and Dr. Junguk Choi. I could not have done this without their full support

and trust.

I also thank my past and present group members, Sejin Kim, Taekon Kim,

Jaeseok Lee, Sushant Gupta, Aniruddh Khanna, Balasundaram Kannan, Jungbae Lee,

and Jinhyung Lee, for their contributions to this research with valuable discussion and

my old and present friends, Jinuk Kim, Sanghyun Eom, Chanwoo Lee, Donghyun Kim,

Kangtaek Lee, Inkook Jun, Donghwa Lee, Kyeongwon Kim, Sungwon Choi, Jihun Choi,

dongjo Oh, Byungwook Lee, Seonhoo Kim, Sangjun Lee, Dongwoo Song, Minki Hong,

Myonghwa Lee who not only helped and encouraged me in this research but also made

my graduate study years lots of fun in Gainesville. I would also like to thank Heesung

Yoon for his helpful research discussion.

I express my most sincere appreciation to my wife and daughter, Minyoung and

Yujin, whose endless love, encouragement and support made me the person I am today.

5

TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES .......................................................................................................... 11

LIST OF FIGURES ........................................................................................................ 12

LIST OF ABBREVIATIONS ........................................................................................... 17

ABSTRACT ................................................................................................................... 20

CHAPTER

1 INTRODUCTION .................................................................................................... 22

Research Rationale ................................................................................................ 22 Scope of the Research ........................................................................................... 24

2 LITERATURE REVIEW .......................................................................................... 28

Chemical Mechanical Planarization (CMP) ............................................................. 28 CMP Process ................................................................................................... 28 CMP Slurry ....................................................................................................... 29

CMP of Dielectrics .................................................................................................. 30 Oxide CMP ....................................................................................................... 30

Process of oxide CMP ............................................................................... 30 Mechanism of oxide CMP .......................................................................... 31 Relationship between particles and wafer .................................................. 31

Shallow Trench Isolation (STI) CMP ................................................................ 32 Process of STI CMP .................................................................................. 32 Mechanism of STI CMP ............................................................................. 33 Mechanism of silicon dioxide using ceria particles ..................................... 34 Mechanism of silicon nitride using ceria particles ...................................... 35 High selectivity ceria-based slurry .............................................................. 37

Ceria Abrasive ........................................................................................................ 38 Advantages of Ceria Abrasive .......................................................................... 38 Disadvantages of Ceria Abrasive ..................................................................... 39

Key Quality Issues .................................................................................................. 40 Removal Rate ................................................................................................... 41 Uniformity and Planarity ................................................................................... 42 Global Planarization ......................................................................................... 42 Selectivity ......................................................................................................... 44 Surface Defectivity ........................................................................................... 45

6

3 EXPERIMENTAL FACILITY AND PROCEDURES ................................................. 58

Introduction ............................................................................................................. 58 Sample Preparation .......................................................................................... 58

Wafers ....................................................................................................... 58 Abrasive particles ....................................................................................... 59 Slurries ....................................................................................................... 60

CMP Equipment ...................................................................................................... 60 CMP Polishers .................................................................................................. 60 Slurry Delivery System ..................................................................................... 60

Characterization and Method .................................................................................. 61 Abrasives .......................................................................................................... 61

Microstructure and shape .......................................................................... 61 Physical properties ..................................................................................... 62 Surface and chemical properties ................................................................ 62

Slurries ............................................................................................................. 63 Electrical potential (Zeta Potential) ............................................................ 63 Particle size distribution ............................................................................. 64

Polished Wafer ................................................................................................. 65 Film thickness measurement ..................................................................... 65 Selectivity between silicon dioxide and nitride ........................................... 66 Oxide CMP within-wafer nonuniformity (WIWNU) ...................................... 67 Defectivity monitoring by wafer defect scattering analysis ......................... 68

4 NOVEL METHOD TO CONTROL THE SIZE OF SINGLE CRYSTALLINE CERIA PARTICLES BY HYDROTHERMAL METHOD AND ITS CMP PERFORMANCE .................................................................................................... 75

Introduction ............................................................................................................. 75 Materials and Methods ............................................................................................ 77

Abrasives .......................................................................................................... 77 Preparation of sol-type ceria precursor ...................................................... 77 Hydrothermal synthesis of ceria particles .................................................. 78

CMP Evaluation ................................................................................................ 78 Preparation of ceria-based slurries ............................................................ 78 CMP tools and consumables ..................................................................... 78

Characterization ............................................................................................... 79 Abrasives ................................................................................................... 79 Ceria-based slurry ...................................................................................... 79 CMP performance ...................................................................................... 79

Results and Discussion ........................................................................................... 80 Preparation of Ceria Particles ........................................................................... 80

Influence of solvent type on ceria particle characteristics .......................... 80 Effect of the precipitation participating anions on nucleation and growth ... 83 Effect of hydrothermal conditions ............................................................... 84

CMP Performance ............................................................................................ 85 Ceria abrasives .......................................................................................... 85

7

Characteristics of ceria abrasive before and after CMP ............................. 87 Polishing performance ............................................................................... 87

Conclusions ............................................................................................................ 91 Synthesis of Ceria Particles by Hydrothermal Method ..................................... 91 CMP Evaluation ................................................................................................ 91

5 POLISHING BEHAVIORS OF SPHERICAL CERIA ABRASIVES ON SILICON DIOXIDE AND SILICON NITRIDE CMP ............................................................... 109

Introduction ........................................................................................................... 109 Materials and Methods .......................................................................................... 111

Abrasives ........................................................................................................ 111 Preparation of as-prepared particles by hydrothermal method ................ 111 Preparation of ceria abrasive particles by solid state reaction (flux

method) ................................................................................................ 111 CMP Evaluation .............................................................................................. 112

Preparation of ceria-based slurries .......................................................... 112 CMP tools and consumables ................................................................... 112

Characterization ............................................................................................. 113 Abrasives ................................................................................................. 113 Ceria-based Slurry ................................................................................... 113 Polishing of wafers ................................................................................... 113

Results and Discussion ......................................................................................... 114 Ceria Abrasives .............................................................................................. 114

Morphological properties .......................................................................... 114 Crystalline structure ................................................................................. 114 Effects of molten salt and as-prepared particle ........................................ 115

CMP Evaluation .............................................................................................. 116 Characteristics of ceria abrasives before and after CMP ......................... 116 Polishing Test .......................................................................................... 117

Conclusions .......................................................................................................... 120 Ceria Abrasives .............................................................................................. 120 CMP Performance .......................................................................................... 121

6 PREPARATION AND CHARACTERISTICS OF THE CERIA COATED SILICA PARTICLES AND ITS CMP PERFORMANCE ..................................................... 134

Introduction ........................................................................................................... 134 Materials and Methods .......................................................................................... 135

Abrasives ........................................................................................................ 135 Preparation of monodispersed silica particles .......................................... 135 Preparation of ceria precursors ................................................................ 136 Preparation of ceria coated silica particles ............................................... 137

Preparation of Ceria-bases Slurry .................................................................. 138 CMP Evaluation .............................................................................................. 138 Characterization ............................................................................................. 139

Results and Discussion ......................................................................................... 140

8

Ceria Coated Silica Particles .......................................................................... 140 Morphology .............................................................................................. 140 Crystalline phase ..................................................................................... 141 XPS spectra of the ceria coating on silica particles .................................. 142 Electrokinetic behavior ............................................................................. 143 Control of thickness of the ceria coating on silica particles ...................... 145 Size control of the ceria coating on silica particles ................................... 145

CMP Evaluation .............................................................................................. 146 Effect of pH .............................................................................................. 146 Effect of down pressure ........................................................................... 147 Wafer roughness (WIWNU) ..................................................................... 147

Conclusions .......................................................................................................... 148 Ceria Coated Silica Particles .......................................................................... 148 CMP Evaluation .............................................................................................. 149

7 SYNTHESIS OF SPHERICAL CERIA PARTICLES BY THERMAL DECOMPOSITION METHOD AND ITS CMP PERFORMANCE .......................... 167

Introduction ........................................................................................................... 167 Materials and Methods .......................................................................................... 168

Preparation of Spherical Ceria Abrasives ....................................................... 168 CMP Evaluation .............................................................................................. 169

Preparation of ceria-based slurries .......................................................... 169 CMP tools and consumables ................................................................... 169

Characterization ............................................................................................. 170 Abrasives ................................................................................................. 170 Ceria-based slurry .................................................................................... 170 CMP performance .................................................................................... 170

Results and Discussion ......................................................................................... 171 Properties of Spherical Cerium Carbonate Precursor .................................... 171

Influence of solvent type on particle morphology ..................................... 171 Effect of dielectric constant on particle morphology ................................. 173

CMP Materials ................................................................................................ 174 Preparation of ceria abrasives ................................................................. 174 Characteristics of ceria-base slurry .......................................................... 175

CMP Evaluation .............................................................................................. 176 Effects of calcination temperature on physical properties of ceria

abrasives .............................................................................................. 176 Effects of suspension pH on oxide and nitride CMP ................................ 178

Conclusions .......................................................................................................... 180 Synthesis of Cerium Carbonates .................................................................... 180 Synthesis of Ceria Abrasives .......................................................................... 180 Preparation of Ceria-based Slurry .................................................................. 181 CMP Evaluation .............................................................................................. 181

9

8 A COMPARISION OF CMP PERFORMANCE IN THE CERIA ABRASIVES SYNTHESIZED VIA VARIOUS METHODS .......................................................... 196

Introduction ........................................................................................................... 196 Materials and Methods .......................................................................................... 196

Sample Preparation ........................................................................................ 196 Preparation of ceria abrasives ................................................................. 196 Preparation of ceria-based slurries .......................................................... 197 CMP tools and consumables ................................................................... 197

Characterization ............................................................................................. 197 Abrasives ................................................................................................. 197 Ceria-based slurry .................................................................................... 198

CMP performance .......................................................................................... 198 Results and Discussion ......................................................................................... 198

Comparison in Polishing Removal Rate ......................................................... 198 Comparison in WIWNU .................................................................................. 199 Abrasive Effects on Defectivity ....................................................................... 200

Conclusion ............................................................................................................ 201

9 CONCLUSIONS ................................................................................................... 207

Solution Growth Abrasives .................................................................................... 208 Grain Control Abrasives ........................................................................................ 209 Core/shell Composite Abrasives ........................................................................... 209 Solid State Abrasives ............................................................................................ 210 Comparison of Polishing Behavior ........................................................................ 212

APPENDIX: DIELECTRIC CONSTANTS OF MIXED SOLUTION OF SOME ORGANIC SOLVENT AND WATER AT ROOM TEMPERATURE ....................... 213

LIST OF REFERENCES ............................................................................................. 214

BIOGRAPHICAL SKETCH .......................................................................................... 220

10

LIST OF TABLES

Table page 2-1 Guiding principles for slurry design in chemical mechanical planarization ......... 57

4-1 Comparison of slurries used in this study ......................................................... 107

4-2 The results of the CMP evaluation. ................................................................... 108

5-1 Comparison of slurries used in this study. ........................................................ 132

5-2 The results of the CMP evaluation. ................................................................... 133

6-1 The results of removal rate for the ceria coated silica particles ........................ 166

7-1 Dielectric constants of mixed solvent, zeta potentials and morphologies of cerium carbonate compounds with the ratio of ethanol to water ....................... 195

8-1 Comparison of ceria abrasives used in this study ............................................. 205

8-2 The results of the CMP evaluation .................................................................... 206

11

LIST OF FIGURES

Figure page 2-1 Schematic for CMP process. .............................................................................. 48

2-2 Schematics of ideal oxide ILD CMP. .................................................................. 49

2-3 Oxide removal mechanism by CMP ................................................................... 50

2-4 Shallow trench isolation (STI) CMP process ...................................................... 51

2-5 Zeta potential of the oxide/nitride substrates and ceria abrasive as a function of pH. .................................................................................................................. 52

2-6 Silicon nitride SN2 hydrolysis reaction scheme. .................................................. 53

2-7 (a) Zeta potential of the oxide/nitride substrates and ceria abrasive as a function of pH and (b) the formation of passivation layer on the surface of STI structure with anionic organic polymer. .............................................................. 54

2-8 The comparison on (a) removal rate for oxide substrate with different abrasives and (b) Mohs hardness of ceria and materials to be polished during CMP......................................................................................................... 55

2-9 The schematics of surface defectivity. ................................................................ 56

3-1 Layout of the SKW-1 pattern wafer: (a) pattern density and pitch size layout, (b) mask floor plan, and (c) cross-sectional view ................................................ 70

3-2 Schematic diagram of rotational CMP tool ......................................................... 71

3-3 Schematic illustration of a slurry delivery system ............................................... 72

3-4 Diagram of film thickness measurement system using NanoSpec. .................... 73

3-5 Schematic illustration of light scattering analysis ................................................ 74

4-1 XRD patterns of ceria particles synthesized from the mixture of water and different alcohols; (a) ethylene glycol, (b) methanol, (c) 1,4-buthylene glycol, (d) ethanol. ......................................................................................................... 93

4-2 FETEM photomicrographs of ceria particles obtained by hydrothermal method using a new type of ceria precursor. ...................................................... 94

4-3 FESEM photographs of ceria particles prepared from the mixture of water and different alcohols; (a) ethanol, (b) 1,4-buthylene glycol, (c) methanol, (d) ethylene glycol. ................................................................................................... 95

12

4-4 (a) Average particle size and (b) crystallites sizes of ceria particles synthesized with different dielectric constants of alcohols .................................. 96

4-5 FESEM photographs of ceria particles prepared with different concentrations of potassium hydroxide; (a) 0.5 M, (b) 1.0 M, and (C) 1.5 M. ............................. 97

4-6 FESEM photographs of ceria particles prepared from different concentrations of nitric acid in hydrothermal conditions at 230 oC for 12 hr. ; (a) pH 4, (b) pH 2.5, (c) pH 0.5 and (d) pH 0.5. ............................................................................ 98

4-7 Crystallites size for ceria particles prepared from different pH at (a) 150 oC, (b) 200 oC and 230 oC. ....................................................................................... 99

4-8 FESEM photographs of the ceria particles prepared with different hydrothermal conditions; (a) pH 3.0 at 220oC, (b) pH 3.0, (c) pH 1.5 and (d) pH 0.5 at 230oC, respectively. .......................................................................... 100

4-9 XRD patterns and the (111) peaks analyzed to confirm grain size of the ceria abrasives dispersed in ceria-based slurry (a) A, (b) B, (c) C and (d) D. ........... 101

4-10 FETEM micrographs and of ceria abrasive with average particle diameters of (a) 62 nm (slurry A) and (b) 232 nm (slurry D). ................................................. 102

4-11 Particle size distribution of ceria-based slurry used in this study. ..................... 103

4-12 FESEM photographs of ceria abrasives (a) before and (b) after oxide CMP process. ............................................................................................................ 104

4-13 Results of CMP field evaluation for removal rate and selectivity. ..................... 105

4-14 Results of CMP field evaluation for within-wafer nonuniformity (WIWNU) of silica film. .......................................................................................................... 106

5-1 Schematic diagram of experimental procedure. ............................................... 122

5-2 FESEM photographs of the ceria abrasives prepared with different calcination conditions; (a) slurry A, (b) B, (c) C and d(c) D ............................... 123

5-3 (a) XRD patterns and (b) the (111) peaks analyzed to confirm crystallite size of the ceria abrasives dispersed in slurry (a) A, (b) B, (c) C and (d) D ............. 124

5-4 The variation of crystallite size as a function of the concentration of grain growth accelerator ............................................................................................ 125

5-5 FETEM micrographs of the ceria abrasives prepared with different cerium precursor; (a) cerium hydroxide, (b) cerium nitride, (c) cerium chloride and (d) cerium dioxide .................................................................................................. 126

5-6 Particle size distribution of ceria slurries as function of abrasive size. .............. 127

13

5-7 FESEM photographs of ceria abrasives (a) before and (b) after polishing ....... 128

5-8 Results of CMP field evaluation for removal rate and selectivity ...................... 129

5-9 TGA curves of the ceria abrasives dried from (a) slurry A and (b) slurry D ...... 130

5-10 Within-wafer non uniformity (WIWNU) of oxide film. ......................................... 131

6-1 FESEM images of silica core particles obtained by modified Stber method. .. 150

6-2 (a) FESEM and (b) HRTEM micrographs for the surface condition of coated particle and FESEM micrographs for ceria coated silica particles prepared by (c) precursor B and (d) precursor A, respectively. ............................................ 151

6-3 XRD patterns of the synthesized particles; (a) bare silica particles, (b) ceria coated silica particles prepared by precursor B, and (c) precursor A ............... 152

6-4 XPS survey spectrum of ceria coated silica particles ....................................... 153

6-5 XPS spectra of O 1s peaks of ceria coated silica particles. .............................. 154

6-6 XPS Ce 3d multiplex of ceria coated silica particles ......................................... 155

6-7 FESEM photographs for ceria coated silica particles prepared at different pH (a) 3.2, (b) 6.8 and (c) 9.7 ................................................................................. 156

6-8 Electrophoretic mobility for (a) silica particles (b) ceria coated silica particles and (c) ceria particles ....................................................................................... 157

6-9 Scheme of the formation mechanism of ceria coated silica particles at different pH. ...................................................................................................... 158

6-10 FESEM micrographs of ceria coated silica particles prepared by different concentration of ceria precursors (a) 0.0 ml, (b) 1.0 ml, (c) 2.0 ml, (d) 4.0 ml and (e) 8.0 ml. .................................................................................................. 159

6-11 The variations of (a) coating thickness and (b) average particle size for samples obtained by changing the concentration of ceria precursors .............. 160

6-12 FESEM micrographs of the silica particles with different size; (a) 105 nm, (b) 214 nm, (c) 332 nm, and (d) 442 nm ................................................................ 161

6-13 FESEM micrographs of the ceria coated silica particles obtained from different core silica particles with different size; (a) 146 nm, (b) 256 nm, (c) 334 nm, and (d) 384 nm. .................................................................................. 162

6-14 Results of CMP field evaluation for removal rate as function of pH .................. 163

14

6-15 Results of CMP field evaluation for removal rate as function of CMP pressure. ......................................................................................................................... 164

6-16 The result for within-wafer non uniformity (WIWNU) of oxide film ..................... 165

7-1 XRD patterns of cerium compositions produced under precipitation conditions with different solvent. ....................................................................... 183

7-2 FESEM micrographs of cerium carbonate compounds obtained by using pure water as solvent ................................................................................................ 184

7-3 FESEM micrographs of spherical cerium carbonate particles prepared from the mixture of water and different alcohols; (a) methanol (CH3OH), (b) ethanol (C2H5OH), (c) 2-propanol (C3H8O), and (d) 1, 4-butandiol (C4H10O2). . 185

7-4 FESEM micrographs of cerium carbonate compounds prepared by various ratio of ethanol to water: (a) 0, (b) 1, (c) 3, and (d) 5 ........................................ 186

7-5 The XRD pattern of (a) cerium carbonate prepared by using mixed solvent of ethanol and water and (b) ceria abrasives obtained from thermal decomposition of the cerium carbonate at 700 oC. ........................................... 187

7-6 FESEM micrographs of (a) as-prepared particles of ceria abrasives and (b) ceria abrasives obtained from thermal decomposition at 700 oC. ..................... 188

7-7 Relationship between surface area and crystalline size of ceria abrasives as a function of calcination temperature. ............................................................... 189

7-8 Electrokinetic behavior of silica, ceria and ceria with surface active agent added as a function of suspension pH .............................................................. 190

7-9 The changes in particle size distribution of ceria-based solvent as a function of suspension pH .............................................................................................. 191

7-10 The CMP evaluation for removal rate of oxide and nitride films as a function of calcination temperature ................................................................................ 192

7-11 Results of CMP field evaluation for removal rate and selectivity ...................... 193

7-12 The CMP evaluation for removal rate of silicon oxide wafer as a function of suspension pH. ................................................................................................. 194

8-1 FESEM micrographs of various kinds of spherical ceria abrasives synthesized by variety methods; (a) hydrothermal method, (b) flux method, (c) surface-induced precipitation method, and (d) thermal decomposition. ...... 202

8-2 Particle size distribution of ceria-based slurries. ............................................... 203

8-3 Comparison of different ceria abrasives on surface defectivity. ........................ 204

15

A-1 The dependence of dielectric constant on the composition of different alcohols and water ............................................................................................ 213

16

LIST OF ABBREVIATIONS

Viscosity

m Micrometer (1 X10-6cm)

A Specific surface area

Angstrom (1 X10-10 cm)

AW Active trench

BET Brunauer-Emmett-Teller

CMP Chemical mechanical planarization

D Crain size

dBET Average particle size determined by BET

DLVO Derjaguin Landau Verwey Overbeek

dSEM Average particle size determined by FESEM

dXRD Crystalline size estimated from XRD patterns

FESEM Field emission scanning electron microscopy

FTIR Fourier transform infrared

h Hour

IC Integrated circuit

IEP isoelectric point

ILD Interlayer dielectric

K Degrees Kelvin

kV Kilo voltage

LOCOS Local oxidation of silicon

17

LPCVD Low-pressure chemical vapor deposition

M Molarity

mA Milliampere

mg Milligram

min Minute

mL/min Milliliter per minute

MRR Material removal rate

nm Nanometer (1 X 10-9 cm)

oC Degrees Celsius

PAA Poly acrylic acid

PECVD Plasma enhanced chemical vapor deposition

psi Pound per square inch

rpm Rate per minute

SSA Specific surface area

STI Shallow trench isolation

TEM Transmission electron microscopy

TG/DTA Thermogravimetric and differential thermal gravimetry

TW Trench width

ULSI Ultra large scale integrate

WIWNU Within-wafer nonuniformity

wt % Weight percent

XPS X-ray photoelectron spectroscopy

18

XRD X-ray diffraction

Half-width of the diffraction peaks

Dielectric constant

Diffraction angle

Wavelength

Zeta potential

19

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

NOVEL SYNTHESIS METHODS FOR PREPARATION OF CERIA ABRASIVES FOR CHEMICAL MECHANICAL PLANARIZATION APPLICATIONS IN SEMICONDUCTOR

PROCESSING

By

Oh Myoung Hwan

December 2010

Chair: Rajiv. K. Singh Major: Materials Science and Engineering

As the device design rule decreased, ceria-based slurries have been widely used

instead of silica-based slurries in a variety of chemical mechanical planarization (CMP)

applications for multilevel integrated circuit (IC) manufacture, since these slurries

address many of the important issues resulting from the use of silica-based slurries.

However, ceria (CeO2) abrasives usually induce higher scratch level than silica particles

due to its cubic crystalline structure, irregular shape, and poor dispersion stability in

slurry. Therefore, this article is intended to establish novel synthetic methods of ceria

abrasives leading to lower scratch level on wafer surface and ultimately present the

direction of CMP abrasive for future technology nodes in order to meet the ever more

challenging defectivity requirements.

To accomplish these aims, this article introduced the 4 types of novel synthetic

methods for the formation of ceria abrasives. The ceria abrasives were synthesized by

solution growth method, grain control method, core/shell composite method, and

thermal decomposition method. In this investigation, the influences of solvent type and

suspension pH on the formation of ceria particles were intensively investigated. The

20

21

size of ceria particles was controlled by adjusting the reaction parameters of each

method without additional mechanical milling and filtration. The relationships between

dielectric property of the solvent and morphological properties were also discussed in

terms of the supersaturation of solution and electrostatic attraction mechanism. The

resultant particles were characterized with field emission scanning electron microscopy

(FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy

(XPS), X-ray diffraction (XRD), Thermogravimetric and differential thermal gravimetry

(TGA/DTG), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller

(BET), light scattering instruments and zeta potential measurements.

In order to investigate the effects of the synthesized ceria abrasives on CMP

performance, CMP tests were carried out with the ceria-based slurry formulated by

dispersing the synthesized ceria particles with anionic organic polymer. The effects of

the synthesized ceria abrasives in CMP slurry were investigated for silicon dioxide and

silicon nitride CMP process. The polishing behaviors of ceria abrasives were discussed

in terms of morphological properties and mechanical abrasion of the ceria particle. In

this CMP evaluation, material removal rate, selectivity, wafer uniformity, and defectivity

of the polished wafer were measured by metrology tools, which are used in current

integrated circuit (IC) fabrication plants in order to support polishing results obtained by

this investigation.

CHAPTER 1 INTRODUCTION

Research Rationale

As the minimum feature size of microelectronic devices deceases, the newly

developed planarization technique and new consumable materials have been utilized in

modern semiconductor fabrication industry.1 To continually satisfy more demanding

devices, chemical mechanical planarization (CMP) has become one of the most critical

semiconductor fabrication technologies because it offers a superior means for global

and local planarization. Global planarization which is essential to produce a multilevel

integrated circuit (IC) device is achieved by reducing topographic variation at the wafer

scale.2 Without CMP, it would be impossible to fabricate complex, dense, and

miniaturizing multilevel IC devices. Over the past years, CMP has significantly

advanced both in the development of more sophisticated processing tools and in the

formulation of novel slurries to further enhance process performance.3,4 Despite these

advancements, the fundamental knowledge of the effects of the numerous CMP

process variables on polishing performance is not clear, due to the lack of

understanding of the substrate to be polished, the slurry that provides the chemistry and

abrasives for mechanical removal and pad interactions involved. The lack of this

understanding is a significant barrier to the development of next-generation CMP

technologies. Therefore, many research has been investigate to understand the nature

of substrate-slurry-pad interactions that occur during the CMP process.

The CMP has been used for interlayer dielectric (ILD) planarization, shallow trench

isolation (STI) structure, and damascene technologies. For these various CMP

processes, the characteristics of slurry particles are critical in determining the

22

planarization performance of CMP. During the past decade, silica (SiO2) particles were

traditionally used as CMP abrasives to remove deposited oxide topography at

technology nodes of 90 nm and higher. Thus, previous studies for the planarization

performance were focused on mechanical abrasion between silica particles and the

substrate to be polished and chemical modification in silica-based slurry in order to

increase the removal ratio of oxide to nitride layer in STI CMP. However, at the 65 nm

technology node and at below nodes, ceria (CeO2) particles are being introduced in a

variety of CMP applications for IC manufacture, since ceria particles have the capability

of achieving higher removal rates and global planarization than silica particles and

ceria-based slurry can more easily be controlled by additives in the slurry formulation.5

Recently, the demand of ceria particles as abrasives has been rapidly increased in

semiconductor fabrication industry. Therefore, the fundamental knowledge for the

characteristics of ceria particles as CMP abrasive is required in order to enhance the

ability of current semiconductor devices.

Compared with other abrasives used in CMP slurry, ceria particles is commonly

used in the ultra large scale integrated (ULSI) circuit structure due to the effective

removal rate for oxide film and the softness of the particles. Recently, ceria-based slurry

has been used in CMP of STI structures consisting of silicon dioxide and silicon nitride

(Si3N4) deposition due to its high selectivity over nitride.6 For the STI-CMP process, the

use of high selectivity slurries is very important to halt the polishing at the nitride stop

layer and reduce the amount of defects such as erosion and dishing. The structural

properties, chemical aspects and morphological characteristics of the ceria abrasives

have been identified as the important parameters that influence the CMP performance

23

such as oxide removal rate, removal selectivity, CMP-induced defects and wafer

uniformity.7 Therefore, many approaches to control these properties of ceria abrasives

have been extensively investigated.

Even though the characteristics of ceria particles significantly affect the quality of

CMP process, it is not easy to manufacture ceria particles as CMP abrasives. The

commercial method for synthesis of ceria particles involves thermal decomposition of

cerium salts such as cerium carbonate and cerium hydroxide. This method leads to very

porous ceria particles with high surface area, inducing softness and high chemical

reactivity to oxide films.8 However, the size and the shape of ceria abrasives are very

limited since particle growth is difficult to control during calcination process. To achieve

the desired particle size and the uniform particle size distribution, mechanical milling

and filteration is required. Other methods for preparing ceria abrasives are liquid phase

processes. These methods can lead to ceria abrasives with desirable morphological

characteristics by manipulating reaction parameters. However, the size of ceria

abrasives is limited to less than 100 nm. Use of these small size particles results in low

removal rates of target layers during CMP. Therefore, a new method to overcome these

problems of ceria abrasives is required.

Scope of the Research

The research presented in this dissertation is intended to establish novel synthetic

methods of preparing ceria abrasives for ILD and STI CMP with precise morphologies

and chemical composition. The overall objective of this research is to investigate the

effects of abrasive material properties on polishing removal rate and wafer defectivity by

using different kinds of ceria particles obtained from a variety of synthesis methods. On

the basis of results obtained from this study, this article will ultimately present the

24

direction of CMP abrasive for future technology nodes in order to meet the ever more

challenging defectivity requirements. To achieve these aims, the research presented in

this dissertation was devoted on a variety of preparation methods of ceria particles. For

CMP performance evaluation, the effect of the resultant ceria abrasives on the removal

rate, the oxide-nitride removal selectivity and within-wafer nonuniformity (WIWNU) was

investigated. A synopsis of the efforts constituting this study is organized as follows.

Chapter 2 reviews the literature on the CMP process including main components

in planarization performance. The procedure and mechanism for ILD and STI CMP was

addressed in detail. From these backgrounds, the need of ceria abrasives was

emphasized in CMP application and motivations for the implementation of the CMP

process using ceria abrasives were discussed. Furthermore, the important issues for

CMP evaluation were summarized and discussed in terms of abrasive characteristics.

New approaches for developing more effective abrasives are introduced.

Chapter 3 was devoted to the studies conducted on the preparation and analysis

techniques of consumables used in this study. For abrasive particles, the synthesized

particles were analyzed by XRD, FESEM, TEM, XPS, FTIR, TG/DTA and BET. For

slurry, slurry stability, mean particle size, and zeta potential measurements was

reported using a variety of light scattering instruments. For CMP evaluation, film

thickness, selectivity, nonuniformity, and defectivity of the polished wafer were

measured by metrology tools using in current integrated circuit (IC) fabrication plants in

order to support polishing results obtained by this investigation. Brief descriptions of

measurement principles for each facility are also presented in this chapter.

25

Chapter 4 investigated the effects of single crystalline ceria abrasives on silicon

dioxide and silicon nitride CMP process. The single crystalline ceria particles were

synthesized by heating peptized ceria sol as precursor under hydrothermal conditions.

In this chapter, the relationships between dielectric property of the solvent and particle

size were investigated in terms of the supersaturation of solute. In addition, the

influences of precipitation participating anions (OH-) and acidic hydrothermal medium on

crystallites size of ceria particles were studied. Furthermore, the polishing behavior of

the single crystalline ceria abrasives was discussed in terms of morphological properties

of the abrasive particle.

Chapter 5 discussed the effects of spherical ceria abrasives on planarization

performance. The ceria abrasives were prepared by the flux method, using potassium

hydroxide (KOH) as the grain growth accelerator. In this chapter, CMP test was carried

out with four types of ceria-based slurry formulated by dispersing the ceria abrasives

with different particle size in order to determine how the removal rate, removal

selectivity, and wafer surface roughness of oxide and nitride films depend on the

abrasive size and particle size distribution in slurry.

Chapter 6 presented studies conducted on the synthesis of monodispersed

ceria coated silica particles and its CMP performance. The coated particles were

prepared by the surface-induced precipitation method, in which a new type of ceria

coating precursor was deposited on the surface of spherical silica particles via

electrostatic attraction route. The ceria coating precursor was synthesized by the sol-gel

technique, which employs ethanol as a solvent. In this chapter, the effects of solvent

type and solution pH on the formation of ceria coating layer were investigated. CMP test

26

was performed with the different types of slurries with 146 nm of abrasive size

controlled by using 135 nm of colloidal silica particles. The suspension pH effects were

investigated as a function of the applied head load and explained in terms of

absorption/repulsion behavior between abrasives and materials to be polished.

Chapter 7 introduced a novel method to synthesize the spherical ceria particles

via two-step procedure. In first step, spherical cerium carbonate particles were prepared

via simple precipitation method using alcohol/water mixed solvent. In second step, the

ceria particles were obtained by subsequent thermal decomposition of the precursor.

After calcination, the resultant particles were used as abrasives of ceria-based slurry

without mechanical milling and filteration. In this chapter, the effects of physicochemical

solvent properties on the crystalline phase, microstructures and morphological

properties of particles were investigated. In addition, the effects of suspension pH in

slurry on polishing performance were discussed in terms of electrostatic repulsive forces.

Chapter 8 investigated the effects of abrasive material properties on polishing

removal rate and wafer defectivity by using different kinds of ceria particles obtained

from previous chapters. In this chapter, the effects of the brittle behavior of ceria

abrasives and particle size distribution of slurry on wafer surface were discussed.

Chapter 9 summarized the conclusions of this study and offered some suggests

for future research.

27

CHAPTER 2 LITERATURE REVIEW

Chemical Mechanical Planarization (CMP)

Chemical mechanical planarization (CMP) is an abrasive process using chemical

agents and a circular action to polish the surface of the wafer smooth. Planarization is

the process of smoothing and planning surface. CMP can be also referred to chemical

mechanical polishing that causes planarization of surface. However, the meaning of

polishing is different from the meaning of planarization. Polishing generally refers to

smoothing the surface not necessarily planar. Thus, the primary function of CMP is to

planarize individual layers in complex integrated circuits. The slurry is the very important

key player among the CMP consumables providing both chemical and mechanical

effects.

CMP Process

A schematic of a typical CMP process is illustrated in Figure 2-1. The wafer is held

on a rotating carrier force down and is pressed against a polishing pad attached to a

rotating disk, while chemically and mechanically active slurries are applied. CMP slurry

contains abrasive silica or ceria particles suspended in an aqueous medium. Both

mechanical action of the abrasive particles and the chemical action of slurry

constituents remove material from the wafer surface. Planarization results because

material is removed faster from protruding regions on the surface than from recessed

regions. The general requirements of CMP can be summarized as follows: First, there is

a need for high removal rates of the material to be polished to achieve the needed

throughput. Second, the selectivity of the slurry must be sufficiently high so that only the

material of interest is polished. Third, the polished surface must exhibit excellent

28

topographical uniformity. Finally, local dishing and erosion effects must be minimized to

satisfy the die-level flatness requirements of sub-0.3 micron devices. Therefore, in order

to meet the requirements, it is necessary to understand in detail the nature of contact

between chemical-mechanical consumable and individual films. Among these

consumable, CMP slurry is one of the most crucial elements to improve the quality of

multilevel interconnect networks. It is generally agreed that CMP slurries should be

designed to optimize for specific applications.1

CMP Slurry

CMP slurry is typically contained with suspended abrasive particles, an oxidizing

agent, corrosion inhibitor, and other additives including dispersants.1 During the CMP

process, the abrasives in the slurry and the rotating polishing pad provide the

mechanical action that removes material on the surface layer. The chemical

components of the slurry accelerate polishing and can be mixed to select specific

substances on the surface of the wafer. The types of CMP slurry is categorized by the

target materials polished in CMP process. Abrasives and chemical components are also

changed by the properties of layers polished during CMP process. Abrasives in the

slurry play the very important role of transferring mechanical energy to target material.

Silica or ceria particles are commonly used as abrasive of oxide CMP process and

alumina particles are used in metal CMP process.1 During CMP process, it has been

known that the abrasive particle size and size distribution have an enormous impact on

the evolution of microscratches. Over the past 10 years, the chemical property of

surface and the hardness of abrasive have been identified as the important parameters,

which affect removal rate, selectivity, and the quality of surface polished during CMP

process. Chemical components in the slurry can be designed for specific functions by

29

the addition of oxidizers or adjusting the pH of the chemical vehicle.9 It is generally

understood that additives modifies the surface to be polished and yields a softer and

porous complex layer, which is then removed by mechanical force in the process.

Moreover, dispersion agent is used to provide a stable dispersion of abrasive. Therefore,

the quality of polishing, which is critical to yield, depends upon the quality and

consistency of the CMP slurry. CMP slurry must continually improve to meet much

higher performance specifications demanded by the trend of new types of CMP

technique and the introduction of noble materials.

CMP of Dielectrics

CMP is commonly employed for both the front and back end processing of

integrate circuit (IC) devices due to its unique global planarization capability. This

process includes interlayer dielectric (ILD), shallow trench isolation (STI), pre-metal

dielectric (PMD), and copper CMP. Slurry is specifically modified for each CMP process

to improve the polishing performance such as removal rate, removal selectivity, global

planarization, and minimized defectivity. Among these CMP processes, this paper will

deal with oxide used as ILD and STI CMP to understand the fundamentals of slurry

design for CMP. In this part, I discuss the procedure and mechanism for these CMP

process.

Oxide CMP

Process of oxide CMP

Oxide planarization is probably the most common of all CMP processes. Inter-level

dielectrics (ILD) are routinely planarized prior to the deposition of the next metal layer.

Oxides layers vary thickness, but generally between 5000 and 10,000 of dielectric

material is removed during CMP. Figure 2-2 shows the schematic for the ILD CMP.10

30

Mechanism of oxide CMP

Oxide removal does not occur as a result of physical abrasive action alone. Rather,

it is the result of chemical reactions in which oxide bonds form between the slurry

particles and the wafer surface.11,12 The physical abrasive action then comes into play

as the moving slurry particles break these bonds and move away. Although much

research is still being conducted in this area, it is known that the process proceeds

along these lines, as illustrated in Figure 2-3:

1. Hydroxylation - formation of hydrogen bonds between oxides on the wafer

surface and the slurry particles

2. Formation of hydrogen bonds between slurry and wafer

3. Dehydration (expulsion of H2O)

4. Breaking of bonds as the slurry particles are forced along

The most common slurries used for ILD CMP are silica-based and ceria-based.

These slurries generally have particles which range in size from 30 ~ 150 nm.

Relationship between particles and wafer

An understanding of the nature of contact between particles and the wafer to be

polished is essential to maintain the strict process requirements for manufacturing

current and future generation integrated circuit (IC) chips. Especially in oxide CMP,

particles play an important role in achieving desired CMP performance such as high

material removal rate, low surface defects, and local global planarization via mechanical

abrasion and chemical modification of the wafer surface.10 In spite of its importance, the

effect that the slurry particles have no polishing performance is not clear. For polishing

of copper or ferrite, it was suggested that the polishing rate is proportional to particle

size and solids loading.13,14 Cook presented data suggesting that the polishing rate is

31

independent of particle size for glass polishing.12 Izumitani suggested that the polishing

rate decreases with increasing particle size.15 Singh suggests two polishing mechanism

in silica CMP.16 One is a contact area based mechanism by which

3/13/10

CA (2-1)

where A is the contact area, C0 is the particle concentration (the number of

particles) and is the particle diameter (abrasive size). In this model, the polishing rate

increases with an increase in particle concentration and a decrease in particle size,

which was observed during tungsten CMP.17 The other is an indentation volume based

mechanism by which

3/43/10 CV (2-2)

where V is the indentation volume. According to this indentation volume based

mechanism, the polishing rate increase with decreasing particle concentration and

increasing particle size. This mechanism was observed via silica polishing experiments.

Shallow Trench Isolation (STI) CMP

Process of STI CMP

The shallow trench isolation (STI) process is one of the most important

applications of CMP. This process has emerged as the primary technique for advanced

ultra large scale integration (ULSI) technologies. This process was developed as an

alternative to traditional local thermal oxidation processes (LOCOS). The LOCOS

process has a major drawback known as the birds beak phenomenon. A birds beak

defect occurs due to the diffusive nature of the oxide growth process.1 As the oxide

grows vertically downward into the underlying silicon, it also grows horizontally to the

sides and underneath the silicon nitride mask, thus encroaching into the active device

32

regions. This becomes more of a problem at geometries below 0.25 m. A secondary

benefit of STI is that it can generally be done faster and at lower temperatures than

LOCOS. Referring to Fig. 2-4, the STI processes begins with the growing of a very thin

oxide layer (100 ~ 200 ) sometimes called the pad oxide. Then, a thicker (500 ~

1,500 ) layer of CVD nitride is deposited on top of the pad oxide. These layers are

then patterned with photoresist and trench is etched into the substrate. After the trench

is etched, a thin oxide layer is grown on the trench sidewalls and bottom to smooth out

the corners and to serve as a liner. Finally, 8,000 ~ 11,000 of CVD oxide is

deposited to fill the trench. This oxide is then planarized using CMP. With the oxide

serving as an etch stop, the nitride layer is then stripped away to expose the active

device regions. One of the key issues of this process is the selectivity of the nitride vs.

oxide. Results ranging from 5:1 to 175:1 have been reported.

Mechanism of STI CMP

STI is a specific CMP application which generally requires the selective removal of

silicon dioxide to silicon nitride on a patterned wafer substrate. In this case etched

trenches are overfilled with a dielectric (silicon dioxide) which is polished using the

silicon nitride barrier film as a stop layer. The process ends with clearing the silicon

dioxide from the barrier film while minimizing the removal of exposed silicon nitride and

trench silicon dioxide. This requires slurry capable of achieving a high relative ratio of

silicon dioxide material removal to silicon nitride removal (high selectivity slurry). Ceria-

based suspensions have received considerable attention in STI applications because of

their ability to achieve high selectivity.18,19

33

Mechanism of silicon dioxide using ceria particles

One mechanism of silica glass polishing using ceria particles previously proposed

by Cook involves proton abstraction from silica followed by reaction with Ce-OH to form

a Si-O-Ce bond.13 Cook described the nature of chemical interaction leading to the

accelerated removal rate with ceria abrasives, listed below:

1. Water penetrates into the glass surface

2. Water reacts with the surface, which leads to the dissolution under particle load

3. Abrasives adsorb some dissolution products and leave from the substrate

4. Some dissolution products redeposit onto the substrate

5. Surface dissolution happens between particle impacts

It is hypothesized that the formation of a strong Si-O-Ce bond leads to break of Si-

O-Si bond on wafer surface because the free energy of the formation of cerium oxide

(Hf = -260 kcal/mole) is much less than the free energy of the formation of silicon

dioxide (Hf= -216 kcal/mole).17,18 Maximum material removal happens when a neutrally

charged ceria particle approaches a silica substrate with negative surface charges to

form surface chemical bonds in aqueous environments.

++ + 2OHMHOHM (pH < pHIEP) (2-3)

+ + HOMOHM (pH > pHIEP) (2-4)

where M-OH is the neutral hydroxyl group and pHIEP is pH value at zero charge

of ceria abrasives (isoelectric point), -M-O- is the deprotonated surface groups and -M-

OH2+ is the protonated surface groups, respectively. Figure 2-5 shows the variation of

zeta potential with pH value for substrates (oxide and nitride) and ceria abrasives. At pH

< pHIEP, silica substrate has a negative surface charge and the ceria abrasives have a

34

positive surface charge, leading to absorption between two materials. On the other hand,

with the increasing pH of the solution, the ceria surface becomes more negatively

charged and the silica surface also has a negative surface charge, leading to repulsion

between two materials. The removal of material from the surface of silica-glass during

the polishing process is attributed to a temporary attachment (through surface chemical

bonds) of ceria particles to the silica-glass surface.

++ OHCeOSiOHCeOSi (2-5)

The material removal occur when a silica tetrahedron structure is broken from the

silica substrate because the strength of Ce-O bonding is greater than Si-O bonding.19

+ CeOSiOSiCeOSiOSi (2-6)

Mechanism of silicon nitride using ceria particles

The proposed mechanisms for ceria-based slurry on silicon nitride substrate may

be slightly more complex than proposed for silicon dioxide substrate.22-24 As shown in

Fig. 2-5, the surface charge of silicon nitride has a functionally difference in acidic pH

region. Particularly, at pH 5 ~ 6, the surface of silicon nitride have a positive charge due

to the presence of protonated amine groups which are not present on silicon oxide

substrate.25-27 However, hydrolysis reactions on the surface of silicon nitride occur

readily in aqueous solutions which liberate ammonia and generate silica-like surface

structures. Such a hydrolysis reaction depicted by Eq. 2-7 would favor formation of

reactive surface silanol groups and surface charge at pH 5 ~ 6 would also become

negative.

32243 436 NHSiOOHNSi ++ (2-7)

35

Therefore, the kinetics of the hydrolysis reaction on silicon nitride affects surface

reactivity and functionality with ceria abrasives. Laarz et al.28 proposed an acidic

catalyzed pathway for hydrolysis of silicon nitride in aqueous environments. Fig. 2-6

depicts hydrolysis reaction scheme for a nucleophilic displacement reaction (SN2) of

silicon nitride, listed below:

1. Protonation of surface amine

2. Coordination of water

3. Concerted water insertion and Si-N cleavage

4. Proton transfer to amine leaving group

5. Continued hydrolysis at Si center

A water molecule can coordinate to the silicon via an SN2 insertion known in

organic chemistry as a nucleophilic displacement reaction (SN2 = substitution,

nucleophilic, bimolecular). In this pathway, an amine is liberated as a leaving group after

the first water insertion. This reaction introduces a hydroxyl group into silicon and the

resulting silicon is more electropositive and sterically less hindered. These electronic

and steric considerations induce that subsequent hydrolysis should proceed more

quickly than the initial water insertion. In this reaction, three proton transfer reactions

occur29: (1) from the solution to a surface amine Si-NR2, (2) from the surface amine to

the adjacent water, and (3) from the protonated silanol to the solution amine. R

represents the subsurface neighboring atom covalently bonded to silicon (most likely

nitrogen). R represents the surface atom covalently bonded to nitrogen (either silicon or

hydrogen). Based on this mechanism, molecules which can compete for surface

36

protons should have an impact on hydrolysis rates, affect Si3N4 surface functionality,

and ultimately influence reactivity with ceria abrasives.

High selectivity ceria-based slurry

The surface potential for oxide and nitride are affected by the suspension pH,

dispersants and organic additives during STI CMP process. In order to improve the

selectivity and uniformity, an anionic acrylic polymer is commonly used to passivate the

surface of the nitride film during STI-CMP, which prevents ceria abrasives from

contacting the film surface. Hirai et al.30 explained the selective absorption mechanism

of acrylic polymers on silicon oxide and silicon nitride layers in water-based system.

They showed that the characteristics of the passivation layer are determined by the

acrylic polymers and suspension pH during CMP process. Moreover, many researchers

reported that the selective adsorption is attributed to the difference in surface charge

between silicon oxide and silicon nitride layers.31-33

Generally, the silicon oxide layer and the abrasives in ceria-based slurry with

acrylic polymer have a negative surface charge at pH 3.0, while the silicon nitride layer

has a positive surface charge at pH 3.0.30-35 Figure 2-7(a) shows the variation of zeta

potential as a function of pH value for substrates (oxide and nitride) and ceria abrasives

including acrylic polymer. Philipossian et al.32 also proposed a selective adsorption

model based on the zeta potential of ceria-based slurry with anionic organic polymer in

terms of high selectivity. Figure 2-7(b) represent the formation of passivation layer on

the surface of silicon nitride layer and electrophoretic behavior of each material with

anionic organic polymer during STI CMP process. The attraction/repulsion reaction

between ceria abrasives and oxide/nitride layers results from the different

electrophoretic mobility as a function of suspension pH. These behaviors affect CMP

37

performance such as material removal rate of substrate and removal selectivity between

silicon oxide and silicon nitride layer. Anionic acrylic polymer is commonly used to

improve the removal selectivity by formatting passivation layer on the surface of the

silicon nitride.

Ceria Abrasive

Abrasives in the slurry play the very important role of transferring mechanical

energy to the surface of substrates during CMP. Among these abrasives, ceria as

abrasive has received considerable attention in CMP process due to its chemical

functions leading to high removal rate, silicon oxide to silicon nitride selectivity, and

lower solid content in slurry. However, there are some problems to be worked out.

Advantages of Ceria Abrasive

As mentioned earlier, ceria particles receiving intense attention as a main slurry

component for CMP process in semiconductor manufacturing industry due to the

effective removal rate for oxide film and the softness of the particles.14 Fig. 2-8(a)

compares the removal rate of oxide layers between fumed silica, colloidal silica, and

ceria as a function of normalized polishing stress.19 The removal rate with ceria-based

slurry is greater than that with silica-based slurry. This is attributed to the fact that ceria

abrasive exhibits a chemical reactivity for oxide layer leading to acceleration of the

removal rate during oxide CMP. As a result, the chemical bonding between ceria and

oxide layer can be rapidly removed by the mechanical force generated by pressed pad

and abrasive, and this physicochemical reaction lead to the high removal rate of a

silicon dioxide film by ceria abrasive. Moreover, the hardness of ceria is lower than that

of substrates to be polished during CMP process as shown in Fig. 2-8(b). From this fact,

it is expected that the scratches on the surface of wafer will be decreased by the lower

38

hardness of ceria abrasives. Additionally, ceria-based slurry has received considerable

attention in STI CMP because of its ability to improve removal selectivity. The oxide-to-

nitride removal selectivity is usually enhanced by adding of an acrylic polymer as

additive to water-based slurry with ceria abrasives. Thus, removal selectivity is affected

by the molecular weight, the concentration of acrylic polymer, and the morphological

properties of ceria abrasive. Furthermore, although ceria is a relatively soft material, it

has long been used to polish harder glass substrates effectively. Compared with as high

as 30 wt % for conventional colloidal silica abrasives and 12.5 wt % for fumed silica

abrasives, ceria-based slurries typically contain less than 1 wt % solid content.36 This

will induce a considerable reduction in manufacturing cost and solid waste discharge.

Therefore, ceria particle can provide excellent CMP performance owing to high

polishing efficiency for silicon dioxide film and lower hardness. Therefore, the ceria

particle as abrasive for CMP slurry has been widely investigated to improve the quality

of CMP process.

Disadvantages of Ceria Abrasive

In spite of many advantages of ceria abrasive, this contains critical disadvantages

leading to serious defects on substrates during CMP process. Usually, commercial ceria

abrasives for CMP slurry were synthesized by thermal decomposition of the cerium salt

such as cerium carbonate and cerium hydroxide.8 This method offers certain

advantages, such as the higher chemical activity and the brittle property of ceria

abrasive due to the high porosity of the surface.35 However, the size and morphology of

the ceria particles are very limited in that particle growth is difficult to control.38-40 A large

number of oversized particles in the distribution tend to give high scratch counts on the

polished wafer.41 Also, these particles need a complicated milling process to regulate

39

the size distribution. In order to overcome this problem, many approaches to control

these properties of ceria particles have been extensively investigated by using liquid

phase processes, such as precipitation method,42 hydrothermal method,43-45 sol-gel

method,46 and electrochemical method.47 These is the attractive methods since particles

with the desired size and morphology can be produced by carefully manipulating

parameters such as solution pH, concentration, reaction temperature, time, and the type

of solvent. Besides, these processes can directly synthesize well-crystallized particles

without post-heat treatment. However, the size of ceria particles synthesized by using

liquid phase process was limited to less than ~ 100 nm. These particles lead to the low

removal rate in the CMP process. Moreover, the ceria abrasive in CMP slurry has easily

sedimented because ceria is too dense to remain suspended in solution. The settling

behavior is the different characteristics of ceria abrasive with respect to colloidal silica.

The specific gravity of ceria and colloidal silica is about 7.13 g/cm3 and 2.2 g/cm3,

respectively.48 The particle settling is much more severe for ceria-based slurry than that

for silica. The sedimentation of ceria abrasive induces an unstable polishing rate for

changeable solid contain during CMP and hard aggregates resulting from poor

dispersion stability creates surface scratches on the polished film. Therefore, the

broader particle size distribution and the sedimentation aggregates have a bad

influence on the quality of the polished films during CMP process. Many approaches

have been extensively investigated to overcome these problems.

Key Quality Issues

The most important issues in slurry performance for CMP relate to removal rate,

global planarity, surface topography (dishing and erosion), surface defectivity (including

roughness, scratches, dents, and delaminating), and particle contamination.10 To

40

develop a methodology for designing slurry formulations, one must be able to

understand the mechanisms active during CMP processing. Table 2-1 lists some of the

most important fundamental parameters that must be optimized in order to achieve

acceptable characteristics in CMP slurry.

Removal Rate

A high removal rate is an essential aspect of a CMP performance. The removal

rate is the amount of material removal by CMP in a given time frame. It is calculated

according to the Preston equation, MRR = KpP0V, where MRR is the material removal

rate, P0 the down pressure, V the relative velocity of water, and Kp a constant

representing the effect of other remaining parameters, and the amount is usually

expressed in /min.49 Removal rates depend on the film being removed, type of pad

and slurry being used, amount of downforce and relative velocity of the wafer carrier

and polishing platen.50-52 Especially, the removal rate of dielectrics can be affected by:51

1. The size and distribution of the abrasives in slurry

2. The number of abrasives

3. The pH of the slurry

4. Pre-CMP film stress

Increasing any of these properties will usually result in an increased removal rate.

However, it has been reported that in some instances raising the pH does not

necessarily increase the removal rate. In fact, for some types of slurries reducing the pH

can slightly increase the removal rate. The removal rate is affected by the size and

concentration of slurry abrasives due to frictional force between the abrasives and the

wafer as mentioned in CMP of dielectrics section. The type of oxide, thermal or CVD,

also has an effect on the removal rate. Another factor which affects the removal rate is

41

the topography on the surface. Material is typically removed at a higher rate on small

and isolated features. On larger or tightly spaced features the removal rate can be

reduced drastically. This will also impact the uniformity of removal.

Uniformity and Planarity

Uniformity and planarity are two closely related but distinctly different topics.

Uniformity is the measure of film thickness (or removal rate) variations across the wafer.

Planarity is more a measure of overall die flatness. In other words, a given wafer may

exhibit acceptable planarity but not be very uniform. All of the process variables

mentioned can have direct effect on uniformity. Controlling the process is the key and

much work and research still needs to be done. Uniformity is the standard deviation of

thickness removal rate measurements and it is expressed as a percentage of the

average thickness removed. Planarity, on the other hand, is simply a measurement of

the degree of flatness and can be expressed as a percentage (planarity across the

wafer) or as a specific number. Both of these issues are complicated by a variety of

factors, such as topography spacing. The topography not only affects uniformity across

the entire wafer, but it can cause problems within each specific IC device.

Global Planarization

Global planarization refers to the ability of the CMP abrasives to rapidly planarize

pattern-dependent and large-scale surface morphologies. The lateral dimensions of

surface topography can range from nanometers to several millimeters, due to large

pattern size or gentle topographic variations. The CMP process is typically conducted

on films deposited on patterned surfaces that as a result have significant surface

topography. The pitch of the pattern (the sum of the width of the patterned lines and the

spacing between them) as well as its density can vary significantly across the die, which

42

results in different local polishing pressures across the patterned surface. The variations

in local polishing pressure lead to varying removal rates and thus to varying amounts of

material removed before global planarization can be achieved. Compared with CMP,

chemical etching decreases surface planarity and increases surface roughness, while

mechanical polishing can enhance the planarity but only at a low removal rate and at

the expense of a poor surface finish. A key condition for global planarization is the

formation of a very thin passivating surface layer that is subsequently removed by

mechanical component of the slurry. The thickness of this layer is commonly under 2

nm. The removal rate of thin passivated surface layer is greater at the highest regions of

the wafer surface than at the lowest regions, due to differences in local pressure in

these regions. If the passivated layer is thinner than the difference in height between the

highest and the lowest regions significant planarization is expected to occur with CMP.

In the case of dielectric CMP, it is generally believed that by controlling the pH in

the alkaline regions, a thin hydrated surface layer is achieved.12 The role of the hydrated

surface layer formed under alkaline pH conditions is to soften the surface so that higher

removal rate can be obtained. It is speculated that the thickness and properties of the

soft, gel-like layer depend on the pH as well as on the contact pressure. The removal

rate of silica under purely mechanical conditions (at low to neutral pH) is less than a

factor of two lower than those obtained under alkaline pH conditions. This indicates that

oxide CMP is more mechanical in nature than metal CMP.

High-planarity polishing is typically observed for slurries that exhibit linear variation

in removal rate with a change in applied pressure (Papp). The planarization capability of

slurry is related to the sensitivity of the removal rate to high and low regions on the

43

wafer. z is the distance between highest and lowest regions, and then the planarity of

the removal rate can be directly related to dRR/dz, or the rate of change in the

removal rate with the variation in surface height. This parameter can be further

expressed as10

=

zddP

dPdRR

zddRR (2-8)

where P is the local pressure on the polishing surface, which is directly

proportional to Papp. The first term in the product is strongly dependent on the

characteristics of the slurry, while the second term is dependent on the mechanical

properties of the pad. The values of dRR/dP are typically enhanced by having a thin

layer that exhibits pressure-dependent material-removal characteristics and harder pad

that transfer a significant portion of the applied pressure directly to the abrasives.

Selectivity

Another important criterion for STI CMP is selectivity, which represents the ratio of

material removal rate (MRR) of silicon oxide to silicon nitride. Generally, a high

selectivity value is desired because the CMP process needs to stop once the silica layer

is removed. To enhance selectivity, the silica layer is typically polished by applying

chemical/mechanical action and ensuring that chemicals do not extend their chemical

assisted synergistic effects to the underlying layer, causing it to be mechanically

removed. For STI CMP, conventional silica-base slurries achieve removal selectivity

value of silica-to-silicon nitride layer in the range of 3 ~ 4.10 These low values can lead

to extensive loss of nitride thickness, especially for large pattern density variations

across the die. Recently, the removal selectivity value has been significantly increased

due to reduced mechanical and chemical effects on the silicon nitride layer with the use

44

of ceria-based slurries. A preferred STI process can be achieved by driving the removal

rate of the protective nitride layer as low as practical while maintaining a reasonable

rate for the fill oxide. Additionally, by suppressing the nitride removal rate, issues

associated with pattern dependent nonuniformity with CMP can be reduced or

minimized. Thus, selected additive and acidic polymer can be added to ceria-based

slurry. Purely mechanical action on the underlying layer can result in high defectivity,

especially in soft materials such as low-k dielectrics. Methods to reduce the mechanical

component of the slurry, for example, by the use of even smaller particles or softer

abrasives, may be required in the future.

Surface Defectivity

Another important aspect of CMP processing is surface defectivity. Defectivity

issues include surface scratches, indentations, surface roughness, dishing, particle

adhesion, and corrosion. Figure 2-9 shows the CMP defectivity for wafer surface.

Among these defects, surface scratches are typical defects of the CMP process and are

produced mainly due to the aggregates of slu